The invention provides for a method of fueling the onboard storage tanks of a vehicle powered by natural gas.
Liquefied natural gas (LNG) fueling for vehicles typically use either a pump or a pressure decant approach. For a pumped system, the LNG bulk storage tank can be maintained at a pressure below that of the onboard tank to be fueled. However, in the pressure decant method, the LNG bulk storage tank is maintained at a pressure significantly higher than the onboard fuel tank. For example, a bulk storage tank for pressure decant may be maintained at 15 barg in order to fuel an onboard storage tank at about 8 barg. This poses a problem when the onboard tank contains vapor or vapor generated during the fueling process which must be returned to the bulk storage tank as condensed liquid. For a pumped system, the vapor can be routed and introduced directly into the bottom of the bulk storage tank where it may be condensed by cold bulk liquid. This is not possible with a pressure decant system, so vapor is often vented to the atmosphere. This practice can result in both economic and environmental inefficiencies.
The LNG fueling process is a periodic process which may operate for only a few minutes with extended time between fueling operations. With a pumped system, it is possible to pre-cool the fueling equipment by circulating liquid through the fueling equipment (e.g., flow meter) prior to fueling. This is not possible with a pressure decant system which can lead to excessive venting and slow fueling.